The invention relates to a method for checking the transmission in an optical transmission installation with optical fibers for conducting optical useful signals between signal sources and signal acceptors, at which optical useful signals are coupled into or uncoupled from the optical fibers, which optical useful signals can be diverted to substitute paths at nodes of the transmission device, wherein for at least one useful signal, checking signals are transmitted between the signal source and signal acceptors and the nodes, and the presence of the checking signals is checked.
The invention also relates to an optical transmission installation, suitable for performing the method, with optical fibers for conducting optical useful signals between signal sources and signal acceptors, at which optical useful signals can be coupled into or uncoupled from the optical fibers, and having nodes of the transmission device, at which nodes switchable branching operations of the transmission path are realized wherein active checking devices are present, which are arranged for transmitting and receiving checking signals transmitted in addition to the useful signals in the respective optical fiber.
Optical glass fiber lines have proved to be especially well suited for low-loss transmission of data with a high information density. Data signals present typically in electrical form are for instance converted into optical signals with light emitting diodes or laser diodes and coupled into a suitable optical fiber line. At suitable points of the line network, the signal is detected with a photo diode, for instance, and converted back into an electrical signal, as which it can be further processed in the usual way. This signal transmission is suitable for overcoming great distances. At suitable intervals, amplifiers and/or regenerators are inserted into the appropriate lines; they are intended to assure that the signal will arrive in readily receivable form at the signal acceptor, formed for instance by a photo diode. As in electrical networks, it is necessary to provide nodes, by which signals are carried to ascertain desired receiver and by which it becomes possible to provide a substitute path for a main line path, in the event that the transmission over the main line path should be impeded. By means of suitably provided bytes in an overhead of the useful signal to be transmitted, automatic substitute line circuits can also be made. A disadvantage of this method is that the selection of a substitute path is possible only within a defined transmission standard for the useful signals, and that in the known system an optoelectronic conversion of the signal is necessary at the ends of the segment protected by a substitute path. These ends may not necessarily coincide with the sources and acceptors of the useful signals.
From European Patent Disclosure EP 0 440 276 B1, it is known, outside the useful signal band, to add a communications signal to the useful signal, by means of optical couplers. Thus control and command signals can be transmitted between nodes of the transmission device. While the transmission of the useful signals takes place in the so-called xe2x80x9cthird windowxe2x80x9d, the xe2x80x9csecond windowxe2x80x9d is provided for transmitting the communications signals. The xe2x80x9cwindowsxe2x80x9d result from the damping properties of the glass fiber material for certain wavelength ranges. In the xe2x80x9cthird windowxe2x80x9d, the damping is minimal, while the xe2x80x9csecond windowxe2x80x9d is formed by a different damping minimum, but in which the low damping values of the xe2x80x9cthird windowxe2x80x9d are not reached. For service communications, the line network thus has its own transmission band available.
The present invention has the object of being able to react quickly and flexibly to line failures in an optical transmission installation, without substantial impairment of parts of the line network that are not affected by the failure.
In keeping with these objects, one feature of present invention resides, briefly stated, in a method for checking a transmission in an optical transmission installation with optical fibers, wherein in accordance with the present invention the checking signals in the respective optical fibers are transmitted in both directions, and in the absence of a checking signal from one line direction, a transmission of a checking signal in the opposite direction is suppressed.
It is another feature of the present invention to provide an optical transmission installation with optical fibers for conducting optical useful signals between signal sources and signal receptors, in which in accordance with the present invention active checking devices are present at the signal sources and signal acceptors and on both sides of the nodes, and if a checking signal fails to be received from one line direction, the transmission of a checking signal in the opposite direction is discontinued.
According to the invention, the signal paths of the transmission devices are motorized in that in the transmission device fibers, checking signals are transmitted that are transmitted and received by checking signal devices. The checking signal devices are provided at least at the signal sources and signal acceptors and on both sides of the nodes. By means of the checking signal devices, line portions are checked for the presence of checking signals in the fibers arriving in the signal direction at the checking device, and checking signals are fed into the outgoing fibers, so that the presence of checking signals can be checked by checking devices located downstream in terms of the signal conduction direction. A prerequisite here is that in the bidirectional transmission devices, the signal transmission in a fiber takes place in only one direction in each case.
If the active checking device ascertains that a checking signal from one line direction is not received, then the transmission of the checking signal from the checking device to the downstream checking device is discontinued, so that the downstream checking device receives a datum about the nonreception of the checking signal at the downstream checking device. The corresponding data of the checking devices can be used as an indicator signal for a switchover within nodes to a substitute path, if such a substitute path exists. If such a substitute path does not exist, then the indicator signal can be used for issuing an alarm.
The active checking devices are preferably connected to a network controller and output a suitable indicator signal to the network control, so that as needed, recourse can be had to higher-ranking provisions to react to the line failure.
For performing the invention, it is especially advantageous if the active checking devices separate the respective arriving checking signal from the useful signal and add a checking signal again to the outgoing useful signal. This assures that a checking signal will only ever have been generated by the last upstream active checking device, so that for instance checking signals from preceding checking devices will not be detected.
The flexibility of the system of the invention becomes especially great if the active checking devices are switched to be passive by the network controller, and the checking devices that have been made passive conduct only a received checking signal onward. The checking devices that have been made passive then have no function of their own any longer. In this way, it is possible to adapt the length of the checking segment being monitored to the current situation, which can be particularly significant in the choice of substitute paths in a network.
In the future, increasingly a plurality of useful signals will be transmitted by wavelength multiplexing over optical fiber lines. For the present invention, it is expedient, for all the useful signals transmitted on one fiber, to form one common checking signal, which selectably contains the checking data for the individual useful signals. Since the useful signals arriving in common on one fiber are typically distributed in nodes to different output fibers, the new checking signal for the applicable output fiber must be reassembled with the checking data for the useful signals to be transmitted, over the output fiber.
The checking and controlling according to the invention of the transmission devices with the aid of the bidirectionally transmitted checking signals is also appropriate for optical transmission installations over which the useful signals are transmitted in only one direction, that is, unidirectionally. All that is needed then is that the checking signals transmitted in both directions over a transmission segment be separated. In an expedient embodiment, this can be done by transmitting the checking signals in the two opposed directions at different wavelengths so that they can be coupled into the fibers for the useful data or decoupled from the useful data via wavelength-selective couplers. If the checking signals are transmitted at the same wavelength in both directions, then they can be decoupled at the ends of the transmission segments, for instance via optical circulators.